Speedy
I may be wrong but reading between the lines I think you may not yet have got yourself a PPL. If this is not the case then forgive me but if it is the case then your first step needs to be to start your PPL training and make sure that you are able to get a Class 1 medical (The CAA will oblige for a trifling £400!).
In order to sit the exams you do not need to be 'signed up' with a flying school but you do need to be signed up with an approved distance learning provider.
The exam test centres are at Gatwick, Oxford, Cranfield (Silsoe), Old Warden (Home of Shuttleworth Collection) and Glasgow.
I think GB is correct. You have 18 months from the time of passing the first exam. This is quite a scary limit for someone like myself working through the course so slowly.
In answer to your final question yes I think it is a good idea. But be prepared for the fact that on top of a full time job it is bl00dy hard work.
I am studying with Bristol, who have a glowing reputation amongst the regulars on this site. However since I have never laid eyes on the material from the competition I can not comment on which is the best. (But I am v happy with Bristol so far).
Cheers